In recent years, plastics have been used in various optical applications such as lenses because they are lightweight, difficult to break, and readily dyed as compared with glasses. Examples of optical plastic materials include poly(diethylene glycol bisallyl carbonate) (CR-39) and poly(methyl methacrylate). However, these plastics have a refractive index of 1.50 or less. Accordingly, for example, in the case where they are used as a lens material, when the diopter increases, the lens becomes thick, thereby reducing the advantage that plastics are lightweight. Especially in a powerful concave lens, the edge of the lens becomes thick so that birefringence or chromatic aberration is generated. In addition, in spectacles, thick lenses are usually not esthetic.
On the other hand, in order to obtain thin lenses, it is effective to increase the refractive index of the material. However, in general, in glasses and plastics, the Abbe's number decreases with an increase of the refractive index. As a result, their chromatic aberration increases. Accordingly, plastic materials having both a high refractive index and a high Abbe's number are demanded.
Examples of such plastic materials include (1) polyurethanes obtained by polyaddition of a polyol having bromine in the molecule thereof and a polyisocyanate (see JP-A-58-164615); and (2) polythiourethanes obtained by polyaddition of a polythiol and a polyisocyanate (see JP-B-4-58489 and JP-B-5-148340). The preferred polythiol as a starting material of the polythiourethanes (2) include branched chains having an increased amount of sulfur (see JP-A-2-270859 and JP-A-5-148340), and polythiols having a dithiane structure introduced thereinto for the purpose of increasing sulfur content (see JP-B-6-5323 and JP-A-7-118390). Further, there are proposed (3) alkyl sulfide polymers containing an episulfide as a polymerization functional group (see JP-A-9-71580 and JP-A-9-110979).
However, with respect to the polyurethanes (1), the refractive index is slightly improved, but there are defects such that the Abbe's number is low, the light resistance is inferior, the specific gravity is high, and the lightweight properties are reduced. Further, with respect to the polythiourethanes (2), those obtained by using a polythiol having a high sulfur content as the starting polythiol have, for example, an increased refractive index as high as from about 1.60 to 1.68, but in comparison with optical inorganic glasses having an equivalent refractive index, the Abbe's number is low, so that the Abbe's number must be further increased. Moreover, with respect to the alkyl sulfide polymers (3), as one example, when the Abbe's number is 36, the refractive index is increased to 1.70, so that lenses obtained by using such polymers may be remarkably thin and light in weight. However, plastic materials having further increased refractive index and Abbe's number are demanded.